This invention relates to planar velocity measurements. Specifically this invention relates to a planar velocity measurement system and method which requires only a single line of sight to measure all three velocity components of a fluid field flow across an illuminated plane.
Systems for planar velocity measurers of fluid field flows are well known. Examples of planar velocity measurement techniques include particle imaging velocimetry (PIV) and Doppler global velocimetry (DGV). In both of these techniques a laser is used to illuminate a fluid flow seeded with particles. With PIV the particle positions at multiple instances in time are recorded with CCD cameras. With DGV the Doppler shift in scattered laser light by particles in the flow is measured.
In general PIV is capable of providing 3-component flow field measurements using a two camera, stereo viewing configuration. An example of a PIV configuration 10 is shown in FIG. 1. Here two cameras 11, 12 view light 18 scattered from a seeded fluid 20 which pass through a pulsed laser illuminated sheet 14. The cameras are positioned in a common horizontal plane and are spatially separated. The accuracy of the out-of-plane velocity component is inversely proportional to the sine of the coupling angle 16 between the two cameras. The minimum error occurs at a coupling angle of about xc2x1forty-five degrees. As a result the technique requires either two separate optical access ports to the fluid flow, or at least one large rectangular optical access port to the fluid flow.
DGV is also capable of providing 3-component flow field measurements. A typical DGV system installation is shown in FIG. 2. The technique 40 requires three receiver systems 42, 44, 46. located at oblique angles from the illuminated measurement plane 48. As shows in FIG. 3, each receiver system 42 includes two CCD cameras 62, 64, which share a common oblique view of the illumination sheet 48 through a beam splitting cube or plate beam splitter 60. The first camera 62 corresponds to a reference camera and views the illuminated flow directly. The second camera 64 corresponds to a signal camera and views the illuminated flow through an Iodine vapor cell 66.
The intensity of the Doppler shifted light passing through the Iodine cell is proportional to the frequency shift of the illuminated particles in the fluid flow 50. Because the frequency. shift is a function of the velocity of the particles in the fluid flow, the Iodine cell 66 acts as an intensity-to-velocity transducer. A velocity component of the fluid flow is derived from the ratio of the signal to reference images from the two CCD cameras 62, 64.
With DGV, the coupling angle between the different receiver systems 42, 44, 46 determines the accuracy of the different velocity components measured. If optical access points to a fluid point are not at oblique angles, poor discrimination in the resolved velocity components will result. Therefore, as with PIV, to achieve accurate 3-component velocity measurements, multiple access ports or very wide access ports are required.
Unfortunately with many flow field velocity measurement platforms such as aerospace propulsion testing platforms, optical access is extremely limited. Limited optical access is typically due to concerns regarding mismatched boundary conditions on walls or due to concerns regarding high thermal or mechanical stresses. In many of these platforms only a single optical access port is available. Consequently there exists a need for a new technique for obtaining planar velocity measurements of fluids which can measure all three components of velocity through a limited access optical viewing port.
It is an object of the exemplary form of the present invention to provide a system and method for obtaining planar velocity measurements of fluid flows.
It is a further object of the exemplary form of the present invention to provide a system and method for obtaining planar 3-component velocity measurements of fluid flows.
It is a further object of the exemplary form of the present invention to provide a system and method for obtaining planar 3-component velocity measurements of fluid flows through a limited access optical viewing port.
Further objects of the present invention will be made apparent in the following Best Modes For Carrying Out Invention and the appended claims.
The foregoing objects are accomplished in an exemplary embodiment of the invention by a velocimetry system and method that includes a single, component DGV receiver system configured to simultaneously acquire PIV image data. In one exemplary embodiment of the present invention, a seeded fluid flow field is illuminated with at least two laser pulses to form an illuminated light sheet. At least one of the laser pulses is generated from an injection seeded laser to satisfy the requirements of a DGV system. The second laser pulse may come from either an injection seeded laser or a standard Nd:YAG laser, for example.
The exemplary receiver system is orientated at an angle which is generally perpendicular to the illuminated light sheet plane. The receiver system includes two PIV xe2x80x9cframe-straddlingxe2x80x9d cameras: a reference camera and a signal camera. The scattered light from particles in the illuminated light sheet are split into two beams by a beam splitter. One beam is imaged by the reference camera. The second beam is imaged by the signal camera after being passed through a molecular filter such as an Iodine vapor cell. The exemplary system enables the acquisition of PIV image frame pairs using both the reference and signal cameras. Each camera in the receiver obtains a pair of image frames responsive to at least two laser pulses. The image frames are processed using PIV processing techniques to acquire the complete 2-component in-plane velocity field of the fluid flow.
In the exemplary embodiment, the image frames from the signal and reference cameras are also processed using DGV processing techniques. The orientation of the receiver perpendicular to the illuminated light sheet enables the DGV system to measure the component of the flow velocity that lies forty-five degrees out-of-the plane of the illuminated light sheet. As a result the DGV velocity measurement can be combined with the in-plane PIV measurements to obtain the full 3-component velocity fields across the illuminated plane, through a single optical access port.